1. Field of the Invention
The present invention relates to a method and apparatus for surface treating a rotary cutting tool or the like by utilizing electrical discharge machining.
2. Description of the Background Art
A surface treating technique such as PVD and CVD is frequently used for treating a surface of cutting tools to coat the surface thereof with TiC, TiN and the like. On the other hand, with regard to a surface treating by way of electric discharge machining, a surface treating for a metal die has been proposed, but no surface treating or coating with regard to machining tools has been proposed. FIG. 12 shows a conventional method and apparatus for surface modification by electrical discharge machining reported in the past (for more information, see Masui et al., "Surface Alloying Treatment by Electrical Discharge Machining", Electrical Machining Technology, Vol. 16, No. 53 (1993)).
Referring to FIG. 12, a workpiece 1 to be surface modified is positioned proximate to an electrode 2 which is held by a spindle 3 that can be moved by a drive (not shown) in a vertical direction. The electrode 2 is disposed within a machining bath 4 which contains a dielectric 5 that includes modifying material powder. A machining power supply 6 provides energy for the machining process.
The following is a list of machining conditions:
Workpiece SKH51(61)
Electrode Copper (15.times.15 mm)
Dielectric Illuminating kerosine
Additive powder Impalpable tungsten powder
Grain diameter 1.3 .mu.micronmRmax.
adding amount 20 g/1000 ml illuminating kerosine
Open voltage 80(V)'
Peak current 2.5, 5, 10, 20(A)
Pulse width 5, 10, 20
Duty factor 0.3 (constant)
In operation, a pulse. voltage is applied between the workpiece 1 and the electrode 2 by the machining power supply 6 to generate electrical discharge. The electrode 2, together with the spindle 3, is servo driven by the drive (not shown) in the vertical direction (Z-axis direction) in the process of machining. Since the dielectric 5 includes impalpable powder of tungsten, electrical discharge causes the base metal of the workpiece 1 to be melted on the surface of the workpiece 1 and the tungsten powder in the dielectric 5 to enter the surface, whereby a modification layer, i.e., tungsten alloy layer, is formed on the workpiece 1 surface. Literature reports that a particularly even modification layer is provided by positive-polarity electrical discharge (electrode negative, workpiece positive). It is also known in the art that a similar modification layer is formed on a metal surface by electrical discharge machining using a dielectric including the powder of silicon, chrome or the like, offering high corrosion resistance and wear resistance.
As another similar method for forming a modification layer on a metal surface, Japanese Laid-Open Patent Publication No. HEI2-83119 discloses a method wherein a powder material for forming a surface layer is provided between an electrode and a workpiece to perform oscillatory electrical discharge machining. In this method, a material for forming a surface layer on the workpiece is provided in a machining gap as powder and oscillatory electrical discharge machining is conducted to prevent the powder of a substance used for surface treatment from fixing, whereby an even modification layer can be provided and the evenness of the machined material surface maintained.